Bottom line: we can't expand org-html-mathjax-options to add in MathJax's cancel.js, etc. Yes, your first "quick-and-dirty" worked just fine. Q: How did you know how to format #+html: \(\require{cancel}\) ? Also, how does the org world know that we're talking about loading MathJax's cancel.js? As you all know I'm not a "developer-class" poster here, so my questions often look dull-witted. Will try the patch once I learn a bit more diff in Emacs lore. . . .

On Thu, Nov 2, 2017 at 9:12 AM, Rasmus <rasmus@gmx.us> wrote:
Lawrence Bottorff <borgauf@gmail.com> writes:

> I'm trying to get a typical Latex strikethrough with MathJax in an HTML
> export, i.e., \cancel{thing} puts line through "thing". This
> <http://docs.mathjax.org/en/latest/tex.html#cancel> tells me how. But I
> don't know how to include this MathJax markup in an org file for HTML
> export. If I forego MathJax -- i.e., #+OPTIONS: tex:dvipng instead
> of #+OPTIONS: tex:t -- \cancel works, but I need to use MathJax. It doesn't
> look like orhg-html-mathjax-options is expandable. . . .

For now this works for me:

    #+html: \(\require{cancel}\)
    \begin{equation}
    \cancel{x = 1}
    \end{equation}

Cf. http://docs.mathjax.org/en/latest/tex.html#tex-and-latex-extensions
You could also autoload the extension, cf. https://github.com/mathjax/MathJax/blob/master/test/sample-autoload.html


That being said, I don’t think extensions were available at the time
#+HTML_MATHJAX was introduced.  We could add extension support, but the
questions would be whether to add them to #+HTML_MATHJAX or make a new
keyword, e.g.

    #+HTML_MATHJAX_EXTENSIONS: ext1 ext2 ... extN

I guess it would default to the "autoload-all" extension.

One issue is that some of them have configuration options, like "noError".

If you want, you can try the attached patch.

Rasmus

--
Vote for proprietary math!